bkernel: add config for nspawn bind mount
    
    We want to keep the old mock bind mount for non rawhide branches, but
    rawhide is using nspawn, so we want to add a directive there to pass
    '--bind' to it to correctly mount the pesign socket directory so kernels
    can be signed for secure boot.
    
    See https://github.com/rpm-software-management/mock/issues/140
    
    Moving forward this could be fixed in mock, in which case we remove the
    nspawn args. Or it could be fixed by pesign moving the socket directory,
    in which case we remove nspawn args and adjust the old mock bind mount
    to the new location. For now, this works around the current crop of
    issues.
    
    Signed-off-by: Kevin Fenzi <kevin@scrye.com>